1. Field of the Invention
The present invention relates to high-speed communication between two aircraft, between an aircraft and a ground facility, or between an aircraft and a satellite. More particularly, the invention relates to the use of apparatus to employ subcarrier multiplexing techniques to modulate and demodulate an optical signal, transmitted between the two entities (transmitter and receiver) in free space.
2. Brief Description of Related Developments
Airborne communication systems currently employ radio-frequency (RF) transmission techniques to transmit information in free space. However, due to RF frequency band limitations (imposed by the U.S. and other governments), such systems are limited to data rates on the order of 1 Gbps.
Demands for higher transmission rates are increasing. Free-space optical (FSO) techniques, in which an optical signal (produced by a laser, for example) is used to carry the information, offer one method for higher rate transmission in the airborne environment.
FSO techniques have been developed for many applications, including terrestrial networks between buildings in metropolitan areas, and deep-space communication systems. Only recently have they been the focus of efforts for airborne applications. Typical FSO techniques employ simple on/off keying in which the data is conveyed by turning the light on and off. More sophisticated techniques, such as pulse-position modulation (PPM), in which the position of a short pulse within a designated time window is used to convey the information, are also under development.
Subcarrier Multiplexing (SCM) is a technique developed to minimize optical dispersion in fiber optic transmissions, whereby multiple channels of data are combined, using RF modulation techniques, and then impressed upon an optical carrier for transmission. An SCM receiver extracts the individual RF channels from the received optical carrier, and then uses RF demodulation techniques to recover the data. SCM has been employed in various applications, including Cable TV distribution (where it is the primary method for transporting the analog and digital TV channels through the optical network) and fiber-based optical communications networks.
The use of SCM techniques in the airborne FSO environment will provide a number of advantages over alternative methods, including: re-use of existing RF solutions in the airborne environment; channelization in the frequency domain, which in turn provides greater flexibility in system design; and scalability in terms of system complexity and cost.